The one-toed horse of today steadily evolved from its five-toed ancestor. which theory of evolution can explain this example?

Horses have genes of having five toes, but as they grow, the other four don;t develop. This is an example of patterning and post-patterning modes of evolution. Horses don't hunt preys, so they don't need claws. Horses develops muscles and "the hoof" for them to cover wide areas and long distances.

Your gonna need to upload an image with the question : )

Sure thing, i’ll you